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How does executive producer Oliver Goldstick sum up the second half of season three? "Mona's back, Toby's been black and Byron may be whack," he tells THR.

[Warning: Major spoilers ahead for season 3B.]

Pretty Little Liars is back for more backstabbing and mind games, but this time, a new Rosewood resident will join a growing list of suspects in the ongoing search for Alison's murderer.

"The season theme has been betrayal in all of 3B, but the people they really trust and love the most are going to give our girls an eye-opening ride this season," executive producer Oliver Goldstick told The Hollywood Reporter.

In a candid chat about the midseason premiere and beyond, Goldstick spoke to THR about Toby's "A" spiral, Mona's reintegration into Rosewood life, a troubled parent-child relationship and a rocky road ahead for Ezra and Aria.

The Hollywood Reporter: How would you describe the first episode back?

Oliver Goldstick: The mantra was: Mona's (Janel Parrish) back, Toby's (Keegan Allen) been black and Byron (Chad Lowe) may be whack. It was a game-changer to implicate a parent as a suspect in Alison's death. We go to some dark places. Thank god the network allowed us to go there, because we're exploring very scary, dark corners we've never explored on the show. We're trying to remind [the viewers]: Don't trust anybody.

THR: Mona attempts to reintegrate back into normal life in Rosewood, but what is "normal" to a person like her, and what is she up to?

Goldstick: The thing with Mona that we can really explore in season 3B: Is she a puppetmaster or is she a puppet? We get to go down some paths where you go wait a minute, is Mona the person orchestrating all of this or is Mona just one of the minions for someone who was much more powerful? Mona is really smart, and it's been a lot of fun to explore that. We like to say that her brain has no off switch. She's very much like Spencer (Troian Bellisario) and, therefore, the two of them get to go head-to-head this season. Mona is a big question mark. From this first episode, the PLLs are put on terror alert. There are questions of whether it's Mona or if there is something much bigger.

THR: With Toby revealed as one of the "A" team members, every scene he now shares with Spencer -- and the other characters on the show, for that matter -- has an added tension to it. How much darker do things get for him as he navigates this double life?

Goldstick: Oh yeah, creepy. With Toby, we wanted to pay something off from the first season; it was all there. When he returned, this kid had an ax to grind. He paid a price going away for a crime he did not commit and taking the rap for it, and these four girls were ancillary to Jenna's (Tammin Sursok) blindness. Therefore, it was all there in season one, and now we're realizing: Has Toby all this time been trying to infiltrate this circle of girls to get what he wants? Is it revenge?

THR: Will we see Toby committing more "A"-like acts in the near future?

Goldstick: Yes. You'll see the tip of his behavior [in the midseason premiere]. There is more coming.

THR: When will Ezra (Ian Harding) find out about his son?

Goldstick: Oh, he will, don't worry. It'll be a major obstacle for "Ezria." For us, it was a great time to explore a more mature problem in this relationship. Having a child would be a profound change for Ezra and will absolutely create some real stress in their relationship.

THR: Rosewood is a town where nothing is as it seems, so is there more to the story?

Goldstick: Yes, of course! Right now, you're still dealing with the reality of how long Aria (Lucy Hale) should keep this secret from him, what's going to happen when it's exposed and will she be implicated as part of the deception because she didn't tell him about his child. It will come out in a way that will have a major impact on the future of their relationship.

THR: Aria's relationship with Byron is tested right off the bat, after more proof surfaces that he could have had a hand in Alison's death. Can you talk about where that's headed?

Goldstick: This is a really devastating thing for Aria, who has come to a detente with her father. The whole thing with Ezra last season, her dad forbidding her to have this relationship, then coming around slowly to accepting that some things you cannot change, now we've got another wedge -- a big wedge between father and daughter. And this one's pretty scary. She knows her father has been duplicitous since the first season, but this is a much bigger lie than Meredith.

THR: It certainly affects more people.

Goldstick: Definitely, and whether her friends will protect him is the big question. It's a juicy way we start the season. It begs the question of the other Pretty Little Liars: What are they not willing to talk about? They think they know now what happened to Alison, and are they willing to destroy Aria's life?

THR: What can you tease about Jason's (Drew Van Acker) journey and what we can be expecting for him?

Goldstick: We're trying to raise the question of why Jason is resurfacing in Rosewood. He's been implicated in the murder of his sister; there certainly was something going on between he and Alison that was dark and twisted. The idea has always been, for us and for the girls as well, to question Jason's actions: We don't know if it's to cover his tracks or is he trying to unearth something to help reveal and expose what happened to Alison. It's an ongoing question for us whether Jason is a bad guy or a good guy. That's an answer that will come in 3B.

THR: Will we see "A" using other avenues, besides the usual text messages, to get their message across?

Goldstick: Yes. We felt this time that this "A" had some new venues for torture.